Polarization mode dispersion (PMD) is one of the major obstacles in high-speed (bit-rates of 10 to 40 Gb/s) long-haul transmissions. In order to mitigate PMD, distributed polarization scrambling in conjunction with forward error correction (FEC) has been proposed in the paper “Experimental Demonstration of Broadband PMD Mitigation through Distributed Fast Polarization Scrambling and FEC” by X. Liu et al. in an ECOC 2004 post-deadline session.
The principle of this proposed method of PMD mitigation is shown in FIG. 3: An optical fiber line 1 for transmission of optical signals at a high bit-rate is arranged between an optical transmitter 2 and an optical receiver 3. A plurality of fast polarization scramblers 4 are successively distributed along the fiber line 1 between successive fiber links 5. The purpose of the polarization scramblers 4 is to provide a scrambling signal performing a periodic polarization change of the optical signals transmitted through the fiber line 1 and thus to shorten the duration of the interference of adjacent bit pulses 7 transmitted with a time spacing Ts reciprocal to the bit-rate (e.g. TB=25 ps) into such a short time that the associated bit errors can effectively be corrected by an error correction scheme (forward error correction, FEC). The polarization scramblers 4 work at polarization modulation rates of some 10 MHz (e.g. fscr=20 MHz). A forward error correction decoder 8 is arranged in succession to the receiver 3 for decoding redundant bits being present in the optical signal in addition to information bits and compensating for transmission errors of the information bits by using the redundant bits.
The optical receiver 3 samples the optical signal at a sampling rate corresponding to the bit-rate of the optical signal. In the presence of fiber PMD, the polarization scramblers 4 generate a time jitter 6 of the bit pulses 7 with a period Ts=1/fscr reciprocal to the scrambling frequency fscr. As a result, the bit pulses 7 are time-shifted in their bit slots according to the time jitter 6, i.e. the bit pulses 7 arrive earlier or later than is the case when no jitter is present. In addition, the actual PMD distortion 9 changing periodically within the scrambling period Ts leads also to a changing eye opening and thus to a variation of the optimum position of a decision gate threshold Uth of the receiver 3 for minimum error probability. As a result, the error probability of the receiver 3 is increased.
Since the envisaged high scrambling rates fscr are beyond the tracking ability of the clock recovery of the receiver 3, the receiver 3 cannot compensate for the jitter 6. Beside this, the changing of the threshold Uth is not taken into account. When increasing the PMD mitigation effectiveness of this scheme in the next future, e.g. when incorporating enhanced FEC (UFEC), the jitter will become the limiting factor.
In the paper “Novel RZ Receivers with Enhanced Jitter and PMD tolerance” by L. Möller et al., OFC 2002 Postdeadline Papers, an optical RZ receiver is described which uses two or more sampling points per bit slot. The decisions corresponding to the sampling points are then combined by a logical OR operation in order to reduce an error probability of the receiver. However, the proposed optical receiver works only with RZ (not NRZ) signals. As in the presence of PMD, even RZ signals are “converted” to broader NRZ-like signals, the proposed receiver is not likely to be applicable in the present case.